David Lewis: Notes & Abstracts

Social conventions are analysed, roughly, as regularities in their
solution of recurrent coordination problems -- situations of
interdependent decision in which common interest predominates. An example
is our [Americans', etc.] regularity of driving on the right: each does so
to coordinate with his fellow drivers, but we would have been just as well
off to coordinate by all driving on the left. Other examples are
discussed; conventions are contrasted with other sorts of regularity;
conventions governing systems of communication are singled out for special
attention. It is shown that the latter can be described as conventions to
be truthful with respect to a particular assignment of truth conditions to
sentences or other units of communication.

A counterfactual conditional has the form: if it were that A, then
it would be that B (where A is usually assumed false). What
does this mean? Roughly: in certain possible worlds where A holds,
B holds also. But which A-worlds should we consider? Not
all; those that differ gratuitously from our actual world should be
ignored. Not those that differ from our world only in that A
holds; for no two worlds can differ in one respect only. Rather, we
should consider the A-worlds most similar, overall, to our world.
If there are no most similar A-worlds, then we should consider
whether some A-world where B holds is more similar to ours
than any where B does not hold.
An analysis of counterfactuals is given along these lines. It is shown to
admit of various formulations. It is compared with other theories of
counterfactuals. Its foundations, in comparative similarity of possible
worlds, are defended. Analogies are drawn between counterfactuals, thus
analysed, and other concepts. An axiomatic logic of counterfactuals is
given.

We ought to believe in other possible worlds and individuals because
systematic philosophy goes more smoothly in many ways if we do; the reason
parallels the mathematicians' reason for believing in the set-theoretical
universe. By "other worlds" I mean other things of a kind with
the world we are part of: concrete particulars, unified by spatiotemporal
unification or something analogous, sufficient in number and variety to
satisfy a principle to the effect, roughly, that anything can coexist with
anything. I answer objections claiming that such modal realism is
trivially inconsistent, or leads to paradoxes akin to those of naive set
theory, or undermines the possibility of modal knowledge, or leads to
scepticism or indifference or a loss of the seeming arbitrariness of
things. But I concede that its extreme disagreement with common opinion
is a high price to pay for its advantages. I therefore consider various
versions of ersatz modal realism, in which abstract representations are
supposed to replace the other worlds; different versions suffer from
different objections, and none is satisfactory. Finally, I consider the
so-called problem of trans-world identity. I stress a distinction between
the uncontroversial thesis that things exist according to many
worlds and the very problematic thesis that things exist as part of
many worlds.